port optr to pycapsule + cleanup and/or provide handles

[RonnyPfannschmidt]

while taking a look at _item.c and item.pyx i remembered that cpython provides opaque handles for pointers

it seems to me that optr should be ported to that

btw, whats the reason for that particular abstraction, at first glance it seems strange, but i lack the context in which it originated

[ThomasWaldmann]

_item.c: _object_to_optr and _optr_to_object

IIRC, code is from @enkore and was meant for passing objects around in-process without copying them (e.g. for sending messages with zeromq).